1. Field of the Invention
The present invention relates to an apparatus for forming a porous preform by extrusion that is used to fabricate an optical fiber.
2. Description of the Related Art
In general, as disclosed in Japanese Patent Application No. 18421/91 for example, in a method of forming a porous preform used to fabricate optical fiber by extrusion, a material mainly containing silica powder is mixed with a binder and water, the resultant mixture is kneaded, thereby forming a plastifiable material, and the plastifiable material is supplied to an extrusion machine to form a porous preform.
When a plastifiable material is extruded by means of an extrusion machine, it is rotated in a direction in which a screw is rotated in a cylinder of the extrusion machine. It is known that a layered defect, i.e. a so-called lamination, is formed in the extruded preform, if the plastifiable material does not stop rotating before it reaches a molding head (Yoichi Motoki, Ceramic Fabricating Process, published by Gihodo Shuppan, 1987, page 45).
As shown in FIG. 1, a conventional apparatus for forming a porous preform used to fabricate optical fiber by extrusion comprises an extruding main body 12 constituted by a cylinder 10 and a screw 11 which rotates in the cylinder 10 to extrude a plastifiable material, a cylindrical body 14 having a length of about 100 mm and connected to the cylinder 10 by an O-ring 13, and a molding head 15 connected to the cylindrical body 14 by the O-ring 13. Since the cylindrical body 14 is interposed between the extruding main body 12 and the molding head 15, the plastifiable material can stop rotating before it reaches the molding head 15.
Unlike other molded components, a porous preform must be prevented from even slight contamination so that the optical property of the final product, i.e. the optical fiber, may not be degraded. For this purpose, in the apparatus for forming a porous preform used to fabricate optical fiber, the metal portion of the extruding apparatus is plated with hard chromium.
If the cylindrical body 14 is plated with hard chromium, the rotation of the plastifiable material cannot be sufficiently stopped therein, since friction between the hard chrome and the plastifiable material is low. Hence, the resultant porous preform used to fabricate optical fiber contains a lamination defect. As a result, when the porous preform is sintered and changed to a transparent glassy rod, voids may be formed therein, due to this lamination defect.